recently acquired a HP 5221B and now added the 6th digit to it (Option 001). Nixie tube courtesy of a German eBay vendor that had a tube for sale right at the day that the counter arrived
HP 1820-0092 and HP 1820-0019 ICs for reasonable money from a surplus vendor. The 1820-0016 is $$$, so kludged in a 74LS75, wiring up only the 4 bits to the nixie driver, not the inverted bits to the printer interface. the 7475 has an inverted polarity latch pin, so I added a 2N3904 with 10k base, 10k pullup to invert the latch signal